The present invention relates to robotic handling systems and, more particularly, to robotic handling systems used in the semiconductor fabrication industry for transporting and transferring semiconductive workpieces, such as semiconductor wafers.
In the semiconductor fabrication industry, a series of discoidal wafers are cut from a starting monocrystalline boule and processed through numerous steps to provide circuit patterns on the surface of each wafer. As a general rule, the processing of the wafers is carried out in a step-wise manner, that is, the wafers are transferred one at a time from one discrete process step to the next. To this end, it is common to store groups of wafers in standard-configuration cassettes. The wafers stored in a supply cassette can be removed one at a time, subjected to one of many process steps, and inserted into the same or another cassette. The cassettes retain the wafers in a spaced, parallel relationship so that insertion or removal of a wafer requires moving the wafer in a direction parallel to its medial plane as it is inserted or removed from its cassette.
Since cleanliness is critical to the successful processing of the wafers and since a large number of steps are usually involved, the opportunities for particulate contamination of the wafers are particularly high with regard to those phases of the overall process that involve the removal and insertion of the wafers into their cassettes and the transfer of the wafers from one process step or work station to another. In an effort to reduce both the opportunity for wafer contamination and the labor costs involved in the handling of the wafers, various types of robot handling systems have been developed for removing a succession of wafers from a supply cassette, transporting the wafers to a work station where they are subjected to some type of processing step, and transporting the processed wafers to a receiving cassette with the wafers subjected to a succession of such removal, transporting, processing, transporting, and insertion cycles.
The robot systems typically operate in a three-dimensional coordinate system that includes a vertically aligned Z axis and an orthogonal, horizontally aligned operating axis, also termed the R or `reach` axis. The R axis is rotatable about the Z axis at an angle .theta.. A representative robot system includes a column aligned along the Z axis and having a horizontally aligned wand assembly mounted atop the column. A motor or other actuator in the column elevates the wand assembly to a selected elevation within its range of elevation. The wand assembly is rotated about the Z axis to any desired .theta. angle by another motor or actuator. A thin, spatula-like wand, also termed an `end effector,` is mounted on a guideway in the wand assembly for bi-directional linear movement on the guideway along the R axis between retracted and extended positions.
In a typical application, three or more wafer cassettes and a work station, such as a visual inspection station, are positioned within the area swept by the extended wand. A controller slews or rotates the wand assembly about the Z axis to an angle .theta..sub.s corresponding to the position of the wafer supply cassette and elevates the wand assembly to the proper elevation. The wand is then extended along the R axis to be placed beneath the first wafer to be inspected. The wand is elevated to contact the underside of the wafer as a vacuum system is enabled to pneumatically engage the underside of such first wafer. The wand is then retracted to remove this first wafer from the supply cassette. When the first wafer is clear of the supply cassette, the wand assembly is rotated from its initial position .theta..sub.s at the supply cassette to the .theta..sub.i corresponding position of the inspection station. The elevation of the wand assembly is varied along the Z axis, as appropriate, to correspond to the elevation of the inspection station, and the wand and the first wafer are extended to the inspection station where the vacuum system is inhibited and the wafer released for inspection. The wand is retracted during the inspection step and waits during a dwell period while the inspection is performed. After the inspection is completed, the wand is extended along the R axis to retrieve the wafer. In a typical post-inspection sequence, the inspected wafer is either provided to an `accepted-wafer` cassette for further processing or to a `rejected-wafer` cassette for reprocessing. Depending upon the disposition of the inspected wafer, the wand assembly is rotated to a .theta. position corresponding to an `accepted-wafer` cassette, i.e., .theta..sub.a, or to a .theta. position corresponding to a `rejected-wafer` cassette, i.e., .theta..sub.r, and the elevation of the wand assembly raised or lowered to the proper elevation. The wand is then extended to deposit the just-inspected wafer in the proper destination cassette. These process steps are repeated on a recurring cyclic basis with the throughput of the robot limited by the maximum linear acceleration and rotary slew rates that the wafer can be subjected to without diminishing the reliable and safe transportation of the wafer to and from the various cassettes and the work station.
Efforts have been made to increase the throughput by providing a single-piece dual wand on the wand assembly with the wands aligned in a 60.degree. to 110.degree. range, for example, relative to one another. The wands are controlled by an additional rotary actuator so that one arm or the other can be co-aligned along the R axis and advanced or retracted along the R axis in the usual manner. It should be noted that the arms do not move independently of one another. That is, movement of one arm causes a corresponding movement of the other arm because of the single-piece construction. Thus, care has to be taken in designing uses of the same to assure that a desired movement of one arm does not result in undesirable movement of the other arm. Moreover, certain movements simply cannot be made. And while the dual-wand approach does increase throughput, it undesirably positions the required additional rotary actuator above the plane of the wafer and subjects the wafer surface to any particulate contaminants shed by the actuator over its operating life.
It will be recognized from the above that a robot system of the nature being described requires relatively complex equipment made up of actuators, drives, etc., to effect the desired operations. This has resulted in such robot systems being somewhat cumbersome.